The present invention relates to the field of electronic blood pressure measuring devices, and in particular relates to an electronic blood pressure measuring device based upon the oscillation method and to an electronic blood pressure measuring device which measures blood pressure by detecting the amplitude of a pulse wave.
According to a prior art type of electronic blood pressure measuring device based upon the oscillation method, a cuff is wound on the upper arm of the person whose blood pressure is desired to be measured, such as a patient, and, after the cuff is pressurized to a certain pressure level, wave form information is detected from the cuff pressure and a pulse wave component contained in the cuff pressure during the depressurization of the cuff is isolated, so that the average blood pressure, the maximum pressure, and the minimum blood pressure of the person whose blood pressure is being measured may be determined from such data. A typical such electronic blood pressure measuring device detects the DC component in the cuff pressure signal when the cuff is gradually depressurized, as exemplarily shown in FIG. 7a of the accompanying drawings, and derives the level differences between the maximum values and the minimum values of the pulse wave components contained in the cuff pressure signal for each fixed time interval W1, W2, W3 and so on as parameters as shown in FIG. 7b, then arranges these parameters H(1), H(2), H(3), and so on for these time intervals in time sequence as shown in FIG. 7c, and then applies a certain algorithm to the variation curve (envelope line) of these parameters (for instance, by determining the average blood pressure from the cuff pressure of the interval in which the parameter is maximum, the maximum blood pressure from the cuff pressure on the high pressure side corresponding to a parameter which corresponds to 50% of the maximum value, and the minimum blood pressure from the cuff pressure on the low pressure side corresponding to a parameter which corresponds to 70% of the maximum value), so as finally to determine the blood pressure.
In such an electronic blood pressure measuring device, conventionally the cuff pressure associated with each interval is determined as the cuff pressure at the beginning or at the end of the corresponding time interval. However, according to such a conventional electronic blood pressure measuring device, since the cuff pressure corresponding to each time interval is read out from the cuff pressure at the beginning or at the end of the corresponding time interval, the time points at which the pulse wave appears as its maximum value and minimum value in each interval are different from the time point at which the cuff pressure is read, and therefore some error has in the prior art existed between the parameter (maximum level difference) and the cuff pressure, thereby reducing the accuracy of blood pressure determination provided by the device.
Further, since the pressure value corresponding to the parameter is either at the beginning or at the end of the time interval of the cuff pressure signal in which the pulse wave component is combined and therefore the beginning (or the end) of the interval may correspond either to the maximum point of the pulse wave or to the minimum point of the pulse wave depending upon the particular instance, the curve of the cuff pressure signal tends to oscillate within the amplitude of the pulse wave component, whereby some fluctuations exist in the cuff pressure value, and some distortion exists in the curve of the parameter, thereby reducing the accuracy.
Considering another aspect of the present invention, there is a conventionally known blood pressure measuring technology, known as the Riva-Rocci-Korotkoff method has been known as a known blood investigation method. According to an electronic blood pressure measuring device based upon this Riva-Rocci-Korotkoff method, after a cuff is wound around the arm of a patient and the cuff is pressurized for stopping blood flow, as the pressure is reduced gradually, the blood starts flowing and a certain distinctive blood sound (the so called Korotkoff sound) is produced, and then subsequently this sound diminishes as further depressurization of the cuff progressively takes place. The cuff pressure at which the Korotkoff sound is started is then determined as being the maximum blood pressure of the patient, and the cuff pressure at which the Korotkoff sound disappears is determined as being the minimum blood pressure of the patient, in determining the blood pressure of the patient.
As another blood pressure measuring technology, inserting a cannulae into the artery of a patient is known as a blood investigative method.
However, according to an electronic blood pressure measuring device based upon the Riva-Rocci-Korotkoff method among such conventional blood pressure measuring technologies, the obtained Korotkoff sound is a very small signal and its frequency range is from 30 Hz to 150 Hz. Thus, there has been a problem that since this frequency range tends to be affected by external noises and oscillation noises, these noises could become a cause of erroneous detection, and such effects have often caused errors in blood pressure measurements in prior art devices for blood pressure measurement.
And on the other hand, according to a blood pressure measurement based upon a direct method such as introducing a cannulae into a blood vessel of the patient, the pressure of an artery is transmitted to an external blood pressure transducer by way of a cannulae filled with physiological saline, and in such a method the length of the cannulae, mixing of bubbles therein, and zero point drifts of the blood pressure transducers could cause errors in blood pressure measurement. These errors can be reduced by proper handling, but such handling requires skill and care, thus requiring certain hard to provide techniques in carrying out proper blood pressure measurements. Furthermore, such direct methods as described above have the serious disadvantages that such invasive procedures inevitably cause pain, discomfort, and mental strain to the patient, and increase the possibility of blood tube pain and infections.